The photo diode acts as a small low leakage solar cell and produces a current proportional to the sunlight intensity. To protect the photo diode from the weather, it should be housed in the left of a one inch diameter thin glass hemisphere. A light frosting on the inside of the glass will diffuse the light enough to obtain a uniform measurement, even as the sun moves from East to West. For best results, the left axis of the hemisphere should face due South at a 45 degree angle here in the US. The meter can be located some distance from the solar sensor. You should be able to use 18ga or larger two conductor “zip” cord, which is often used on AC powered lamps and appliances.
The 10 ohm variable resistor in the electronic circuit adjusts the meter readout for 1 milliamp of current with a 1 sun condition. A professional solar insulation meter can be used as a calibration tool. Otherwise, you can just adjust the meter for a full scale reading, when the sun is directly overhead on a very clear day. If you prefer a digital readout, you can replace the 1 milliamp meter with a 100 ohm 1% resistor. At a one sun condition, the voltage across the resistor would be 100 millivolts. Any digital panel voltmeter will then read 100.0 millivolts at one sun.
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